In general, fermentation products are produced by degradation of starch-containing material into fermentable sugars by liquefaction and saccharification followed by conversion of the sugars directly or indirectly into the desired fermentation product using a fermenting organism.
For example, many carboxylic acids are produced industrially on a large scale. They are also pervasive in nature. Carboxylic acids are used in the production of polymers, pharmaceuticals, solvents, and food additives. Industrially important carboxylic acids include acetic acid (component of vinegar, precursor to solvents and coatings), acrylic and methacrylic acids (precursors to polymers, adhesives), adipic acid (polymers), citric acid (beverages), ethylenediaminetetraacetic acid (chelating agent), fatty acids (coatings), maleic acid (polymers), propionic acid (food preservative), terephthalic acid (polymers). Lactic acid is widely used in food, pharmaceutical and textile industries. It is also used as a source of lactic acid polymers which are being used as biodegradable plastics. The physical properties and stability of polylactides can be controlled by adjusting the proportions of the L(+)- and D(−)-lactides. Optically pure lactic acid is currently produced by the fermentation of glucose derived from corn starch using various lactic bacteria (Kenji Okano, Qiao Zhang, Satoru Shinkawa, Shogo Yoshida, Tsutomu Tanaka, Hideki Fukuda, and Akihiko Kondo (2009). Efficient production of optically pure D-lactic acid from raw corn starch by using a genetically modified L-lactate dehydrogenase gene-deficient and -amylase-secreting Lactobacillus plantarum strain. Applied and Environmental Microbiology, 75, 462-467).
However, the lactic bacteria have complex nutritional requirements and the use of corn as the feedstock competes directly with the food and feed.
Feedstocks not competing directly with food and feed are naturally resistant lignocellulosic biomass. Lignocellulose is the non edible part of plants. Using lignocellulose to produce carboxylic acid avoids competition with the food industry. Lignocellulose is the most abundant renewable biomass. The yield of lignocellulose can reach approximately 200 billion metric tons worldwide per year (Zhang, Y. H. P., S. Y. Ding, J. R. Mielenz, J. B. Cui, R. T. Elander, M. Laser, M. E. Himmel, J. R. McMillan, and L. R. Lynd. (2007). Fractionating Recalcitrant Lignocellulose at Modest Reaction Condition. Biotechnology and Bioengineering. 97(2):214-23). Lignocellulose is a feedstock with a low production cost. Lignocellulose can be found everywhere and is available as waste biomass, including agricultural residues (wheat straw, sugarcane bagasse, and corn stover), energy crops (switch grass), and municipal solid waste (paper and paperboard products) (Hamelinck, C. N., V. H. Geertje, and A. P. C. Faaij. (2005). Ethanol from Lignocellulosic Biomass: Techno-Economic Performance in Short-, Middle- and Long-Term.” Biomass and Bioenergy 28(4):384-410.).
However, the characteristics of lignocellulose have different disadvantages for the usage as feedstock not competing with food and feed. Because lignocellulose is mainly made up of lignin, hemicellulose, and cellulose fibers this combines to form a firm, very compact network structure. In a natural state, after size reduction, the access to cellulose is still blocked by lignin and hemicellulose because of the intact cell wall structure. Moreover, cellulose has a highly crystalline structure that is very difficult to break down (Hsu, T. A., M. Ladisch, and G. Tsao. (1980). Alcohol from Cellulose. Chem. Technol. 10(5):315-19.). Therefore harsh pre treatment processes are currently used to destroy the structure of lignocellulosic biomass plant cell walls and make cellulose more accessible to the subsequent process of hydrolysis (during hydrolysis, cellulose is broken down into simple sugars).
Pre treatment methods can be categorized into four types: physical methods (e.g., milling and grinding); physicochemical methods (e.g., steam explosion or hydrothermolysis); chemical methods (e.g., using acids, alkali, oxidizing agents, or organic solvents to treat biomass); and biological methods (e.g., using microorganisms and fungi to treat biomass) (Kumar, P., D. M. Barrett, M. J. Delwiche, and P. Stroeve. (2009). Methods for Pretreatment of Lignocellulosic Biomass for Efficient Hydrolysis and Biofuel Production. Industrial & Engineering Chemistry Research 48(8):3713-29.). However, only those pre treatment methods that employ chemicals currently offer the high yields and low costs vital to economic success. Among the most promising are pre treatments using dilute acid, sulfur dioxide, near-neutral pH control, ammonia expansion, aqueous ammonia, and lime, with significant differences among the sugar-release patterns.
Although pre treatment of naturally resistant cellulosic materials is essential, if high yields from biological operations should be achieved; this operation is projected to be the single, most expensive processing step. For example about 20% of the total cost for the production of ethanol are based on the pre treatment process (Bin Yang and Charles E. Wyman. (2008). Pre treatment: the key to unlocking low-cost cellulosic ethanol. Biofuels, Bioprod. Bioref. 2:26-40). Moreover, pretreatment of lignocellulosic biomass will generally release extractives and other natural products and can form degradation products, such as lignin fragments and derivatives thereof, which are inhibitory or even toxic to downstream enzymes and organisms (Bin Yang and Charles E. Wyman. (2008). Pre treatment: the key to unlocking low-cost cellulosic ethanol. Biofuels, Bioprod. Bioref. 2:26-40). This will reduce the productivity of any subsequent fermentation process with pretreated lignocellulosic biomass as feed stock, if approaches to circumvent this are not used.
Therefore, the availability and use of novel feedstocks for the production of carbon-based chemicals like carboxylic acids would be highly advantageous.